Interior magnet machines have the following characteristics.
First, many interior magnet machines have lower power density than surface mounted magnet machines. The surface area of the magnet is usually reduced when buried, requiring a larger motor or generator to obtain the same output power. The larger size motor or generator can cause packaging or performance problems in the final application.
Second, a trapezoidal air-gap flux distribution is usually generated by a interior magnet rotor. In applications where the winding currents are sinusoidal, the trapezoidal flux distribution results in significant torque ripple. The torque ripple contributes to noise and vibration in the final application. This can be minimized by selection of the correct slot and pole number, but this solution is not always practical.
Third, the abrupt transitions in the rotor flux distribution contribute to cogging torque. Techniques typically used to reduce cogging torque, such as skewing, result in lower power density.
Fourth, interior magnet machines have higher average inductance than surface magnet machines. The higher inductance reduces the power factor of the machine during operation, increasing the complex power (VA) required from the drive to produce a given output torque. Increasing the drive volt-ampere requirement can increase the drive cost if larger power devices must be used.
The output torque of an interior permanent magnet machine is proportional to the back-emf and winding current when the two are in phase. The winding current in a fixed bus voltage system is limited by the back-emf and machine resistance and inductance. A rotor geometry that results in higher back-emf or lower inductance allows the number of turns to be adjusted to obtain minimum current draw. The decrease in current may allow for the use of smaller power devices, reducing system cost.
Prior art solutions for interior magnet machines with power density greater than or equal to surface magnet machines include “V” magnet and spoke magnet designs. The designs can be difficult to magnetize and tend to have high cogging torque.
Prior art solutions for reducing the impact of a trapezoidal rotor flux distribution include machines with distributed windings. Stators with distributed windings tend to be larger than single tooth windings due to the end coils, and may not fit in the package required by some applications. Single tooth windings in which the number of electrical degrees per slot is not equal to 120 or 240 can also be used. The number of practical combinations is limited by the size of the machine.
Prior art solutions for reducing the cogging torque include shaping of the stator and rotor air-gap surfaces and skew. These solutions tend to reduce the power density of the machine.
Prior art solutions for reducing the average inductance of a interior magnet machine include adding slits to the rotor pole cap. These slits are placed perpendicular to the magnet surface in most cases.